In the field of display devices used for image display, represented by CRT displays (CRT), liquid crystal displays (LCD), and plasma display panels, much has been expected in recent years of large high-definition screens such as high vision television screens.
CRTs, which are the most widely used among these display devices, are superior with regard to resolution and image quality, however are not suitable as devices for large screens of 40 inches or more due to their weight and depth. On the other hand, although LCDs have superior properties of low power consumption, and low driving voltage, the production of large screen LCDs is difficult, and LCDs have limited viewing angles. Compared with these devices, as well as PDPs having a wide viewing angle range, the production of large screen PDPs with shallow depth is relatively simple, and developments have already been made in 40 inch class products (for example, Kinouzairyou (Function & Materials) February 1996, volume 16, page 27).
A PDP is a type of gas discharge panel, and generally has a construction in which a front glass substrate and a back glass substrate are arranged facing each other via barrier ribs.
A plurality of conductive metal electrodes made of Ag and Cr/Cu/Cr and the like are arranged in parallel lines on the surface of the front glass substrate which faces the back substrate, and a dielectric layer which covers and insulates the metal electrodes, is formed, which is then covered with a protective layer made of MgO or the like. Also, on the surface of the back glass substrate which faces the front substrate, a plurality of metal electrodes are arranged in parallel lines, a dielectric layer is formed covering the electrodes, and, further, as well as barrier ribs being aligned on the dielectric layer, phosphor layers are applied onto the dielectric layer between the barrier ribs. A discharge gas, which includes rare gas or the like, is enclosed between the front glass substrate and the back glass substrate. When driving the PDP, ultraviolet light is emitted by the enclosed discharge gas as a result of applying a pulse voltage to the pairs of electrodes formed on the front glass substrate, and the ultraviolet light causes the phosphor layers provided on the back glass substrate to excite and irradiate. The light irradiated from the phosphor layers passes through the dielectric layer and the front glass substrate and the like, and is visible to a user. Here, the dielectric layer formed on the front glass substrate is generally constructed from glass having a low fusion point, the properties of such glass including high transparency, a baking temperature of 500–600° C., and sufficient voltage resistance.
Conventionally, Pbo or Bi2O3 glasses which have sufficient voltage resistance have been used as dielectric layers. However, as the relative permittivities of these glasses were high at 10–12, there was a tendency for a large amount of current to flow during discharge, and subsequently power consumption of the PDP tended to increase.
Therefore, the use of SiO2, which has a comparatively low relative permittivity, as the dielectric layer was considered. However, because an SiO2 film is formed by an evaporating method or a spattering method, as well as forming the film to the required thickness (20–30 μm) being difficult, the formed film was easily cracked, as it was difficult to insure a sufficient pressure resistance.
Meanwhile, Na2O—B2O3—SiO2 glasses and Na2O—B2O3—ZnO glasses which do not include the abovementioned PbO, Bi2O3 and the like, and have relative permittivities which are lower than for conventional glass, and softening points of 500–600° C. are being developed for use as the dielectric layer (for example, as mentioned in Japanese Laid-Open Patent Publication No. 9-199037 and Laid Open Patent Publication No. 9-278482). These glasses include alkali metal oxide components such as Na2O (sodium oxide), K2O (potassium oxide), Li2O (lithium oxide) to lower the glass softening point, and because the glass softening point is lowered due to the inclusion of such components, baking of the dielectric layer can be performed at a comparatively low temperature.
However, when this kind of glass, which includes a component which lowers the softening point, is used in a dielectric layer there is a possibility that yellowing will occur on the dielectric layer, front glass substrate and the like. The mechanism which causes such yellowing is considered as follows.
Display electrodes provided on the front glass substrate are made using Ag, Cu or the like, and there are cases where, at the time of baking which is performed to form the dielectric layer, Ag, Cu or the like ionizes and starts to melt and diffuse into the dielectric layer, the front glass substrate and the like. The diffused Ag and Cu ions, which are easily deoxidized by alkali metal ions such as Na ions which are ionized by the component which lowers the softening point, and Sn ions (valency 2) which are included in the front glass substrate, become colloid in such a case. When Ag and Cu have become colloid, the dielectric layer and the front glass are stained yellow or brown, that is to say, yellowing occurs (for example, J. E. SHELBY and J. VITKO Jr Journal of Non Crystalline Solides vol50 (1982) 107–117). In PDP, because such stained glass absorbs light with a wavelength of 400 nm, problems such as decreased blue color brightness and deterioration in chromaticity occur. Further, as Ag and Cu colloids and the like are conductive, they reduce the dielectric resistance of the dielectric layer, and as the colloids are formed from deposits of colloid particles which are far larger than ions, the Ag and Cu colloids are the cause of a reflection of light which should pass through the dielectric layer, which consequently reduces PDP brightness.
In order to restrict this kind of reduction in dielectric resistance and yellowing of the dielectric layer, a possible method is to construct a double-layered dielectric layer by using PbO glass, which does not include a component which lowers the softening point of the glass, to encase the parts of the dielectric layer which are in direct contact with the display electrodes, and then laminate the PbO glass with low-permittivity Na2O—B2O3—SiO2 glass. However this method is not desirable as it increases the number of construction processes for the dielectric layer, therefore increasing construction cost. Further, from an environmental conservation viewpoint, there is also a demand for a construction method which does not use lead (Pb).
In view of the abovementioned problems, the object of the present invention is to provide a highly reliable plasma display panel that includes a dielectric layer which, in order to lower permittivity, does not include PbO, and in which yellowing of the front glass substrate, the dielectric layer and the like is restricted, and dielectric breakdown does not occur; and a production method for the PDP.